1. Field of the Invention
The present invention relates to a radio frequency (RF) power divider of a high output amplifier usable in an ultra high frequency band of a wireless transmitter, and more particularly, to a Wilkilson power divider detecting an AC signal by a metal capacitor, thereby minimizing depletion and stabilizing transmission of signals.
2. Discussion of the Related Art
Recently, in view of the increased diversity and high quality of wireless call services, the importance of maintenance and repair in the market of wireless call services has been recognized. As a demand for the wireless call service is on the increase, the required number of wireless call transmitters for transmitting signals by air path has increased. If various signals are simultaneously transmitted and such signals transmitted by the wireless call transmitter reach certain wide regions, it is possible for service providers to meet the increased demand for the wireless services with the existing personnel and equipment using a high output amplifier that can transmit signals to the wide regions.
However, a related art amplifier amplifying wireless signals has the limitation of a low output power level. To overcome this limitation, a plurality of low or medium output amplifiers connected in parallel are used to obtain the desired high power output.
A device for dividing and/or combining the wireless signals is called a power divider or a power combiner. A power divider is an interdependent device calculating a vector sum total of at least two signals.
RF power dividers can be classified into three categories: T-junction power divider, Wilkilson power divider, and Quadratue hybrid power divider. Among them, a particular power divider is selected according to desired characteristics.
A binary power divider is classified, in frequency of a microwave region, into a micro strip or strip line Wilkilson power divider.
The Wilkilson power divider is formed by connecting cascaded λ/4 converters in series. The cascaded λ/4 converters convert the impedance of an input signal to a parallel structure of a multi-output. An input voltage standing wave ratio (VSWR) of the power divider varies depending on the quality of the impedance converter and the VSWR of load terminating the output of the device.
A simple Wilkilson power divider is useful for a region using a limited bandwidth. But, if it is desired to increase the bandwidth of the power divider, it is required to additionally provide a λ/4 converter and an isolation resistor. That is, to increase the bandwidth, a plurality of converters have to be provided. But this increases the size of the power divider and energy loss due to resistance.
Meanwhile, a “coupling” refers to a state when an energy is interchanged between adjacent objects. Fundamentally, an RF device manages a high frequency signal, which is a frequency signal that is apt to radiate as external electromagnetic waves, so that some energy radiates from a line in electromagnetic waves or magnetic waves. That is, an energy signal flows into the lines of the electromagnetic/magnetic waves directly or indirectly, which is called the coupling. As the frequency becomes higher, the electromagnetic wave energy from the line increases, thereby increasing the coupling. A “coupler” refers to a circuit structure using the coupling and is provided generally in RF devices.
In this sense, a power divider is a coupler that divides one electric power signal into at least two electric power signals. Here, the coupling usually means that the energy is interchanged between disconnected lines, and the coupler means that the signal is interchanged in state that the lines are not connected in DC. In general, the coupling ratio of a coupler relates to a length of parallel lines and an interval between the parallel lines. The length of the parallel lines is usually λ/4 (quarter wavelength), and the interval between the parallel lines is controlled to form a coupler of 3 dB, 6 dB, 10 dB, 20 dB, etc.
For example, if the electric power is divided in half, the coupler is a 3 dB divider (3 dB is twice, and −3 dB is half, herein, ‘−’ is not shown). If the electric power is divided in twenty to one, the coupler is a sampler obtaining a signal.
A coupler has the advantage of receiving the signal without any reflection when the VSWR of input and output terminals is “1” in theory. Thus, the coupler can be used as input and output matching means for an active circuit. At this time, the circuit is divided into two, thereby increasing the size of the circuit. Also, the divided circuits are conversely connected, so that the coupler is served as a connector. Generally, the coupler functions as an input terminal, an output terminal, a coupling port and an isolation port.
The size of the coupler relates to an extracted signal value, especially, to a certain frequency wavelength (usually 4/1λ), thereby narrowing the bandwidth of the signal. The above problem may occur in the Wilkilson power divider.
A related art coupler is used to divide and extract a signal power, which can be variously applicable. However, a related art Wilkilson power divider has the following problems.
First, in the related art Wilkilson power divider, each electrode of a capacitor on the substrate of the divider is formed of a polysilicon layer. Accordingly, as a voltage difference between the electrodes of the capacitor increases, a depletion generated between the electrodes is deepened, thereby deteriorating a voltage coefficient characteristic of the capacitor in the power divider.
Furthermore, in the related art Wilkilson power divider, if a metal capacitor detects an output signal by a contact area formed at a drain region of the substrate for connecting with an upper metal line, an AC signal is detected. However, with the related art Wilkilson power divider, it is hard to detect an original signal such as AC+DC signal.